Intelligence and the capacity for abstract thought evolved in our prehistoric ancestors living in Africa between 50,000 and 500,000 years ago, who relied on their wits to build shelters and hunt prey.

But in more civilised times where we no longer need to fight to survive, the selection process which favoured the smartest of our ancestors and weeded out the dullards is no longer in force.

Harmful mutations in our genes which reduce our "higher thinking" ability are therefore passed on through generations and allowed to accumulate, leading to a gradual dwindling of our intelligence as a species, a new study claims.

Prof Gerald Crabtree, a developmental biologist at Stanford University, explained in the Trends in Genetics journal that a mutation in any one of 2,000 to 5,000 particular genes could lower our intellectual and emotional ability.

Our development of intelligence genes "probably occurred in a world where every individual was exposed to nature's raw selective mechanisms on a daily basis," he said, but the same pressures do not apply today.

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The development of agriculture thousands of years ago led to larger community life, and "the need for intelligence was reduced as we began to live in supportive societies that made up for lapses of judgment or failures of comprehension," he said.

"A hunter-gatherer who did not correctly conceive a solution to providing food or shelter probably died, along with his or her progeny, whereas a modern Wall Street executive that made a similar conceptual mistake would receive a substantial bonus and be a more attractive mate. Clearly, extreme selection is a thing of the past."

Based on the rate at which harmful mutations in our genes happen, and the particular susceptibility of those genes related to intellectual and emotional function, Prof Crabtree calculated that humans "reached a peak" 2,000 to 6,000 years ago.

"I would wager that if an average citizen from Athens of 1000 BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions, with a good memory, a broad range of ideas, and a clear-sighted view of important issues," he said.

Within 3,000 years from now it is likely that all humans will have undergone at least two further genetic mutations which lower their intellectual or emotional stability, but science will most likely have progressed so far that we will be able to solve the problem, he added.

"One does not need to imagine a day when we could no longer comprehend the problem, or counteract the slow decay in the genes underlying our intellectual fitness, or have visions of the world population docilely watching reruns on televisions they can no longer build.

"It is exceedingly unlikely that a few hundred years will make any difference for the rate of change that might be occurring."

Prof Robin Dunbar, an anthropologist at Oxford University, said: "[Prof Crabtree] takes the line that our intelligence is designed to allow us to build houses and throw spears straighter at pigs in the bush, but that is not the real driver of brain size.

"In reality what has driven human and primate brain evolution is the complexity of our social world [and] that complex world is not going to go away. Doing things like deciding who to have as a mate or how best to rear your children will be with us forever.

"Personally I am not sure that in the forseeable future there is any reason to be panicking at all, the rate of evolution with things like this takes tens of thousands of years...no doubt the ingenuity of science will find solutions to these things if we do not blow ourselves up first."